The present invention relates to a novel explosive composition useable as an enhancer agent (transfer charge) for use in a gas generator for an automobile airbag system. The explosive composition of the present invention is characterized in that it has an automatic ignitability, while maintaining a high calorific value.
An airbag system is a occupant restraint system s which has been widely adopted in recent years to improve safety of occupants in an automobile. The airbag system operates on the principle that a gas generator is operated under control of signals from sensors detecting a collision, to inflate an airbag so as to cushion the shock of the occupants from the collision.
The gas generator is operated through the order that the igniter is ignited when it receives the signals from the sensors, first, and then the ignition is transferred to the enhancer agent to make the gas generants ignite.
The enhancer agents serve to ignite the entire gas generant within a fixed time. This permits the gas generator to exert its full performance without any ignition lag as calculated.
Among the existing enhancer agents, the so-called xe2x80x9cBKNO3xe2x80x9d containing boron and potassium nitrate as main components is in general use as the enhancer agent. This enhancer agent is in heavy useful in terms of the advantages that it can ignite in a moment of time and also generates high calorific values and that it generates metal thermo-particles of boron to accelerate the ignition. It has the disadvantage, however, that the number of moles of the generated gas is 0.4 or less per 100 g of the BKNO3, so that when this enhancer agent is used for a gas generant of poor ignitability, a reduced amount of gas is generated, and as such can cause unstable ignition.
On the other hand, for the purpose of weight saving of the gas generator, aluminum is now in widespread use as a container material of the gas generator, instead of stainless steel (SUS). The conventional container made of SUS is excellent in strength in high temperature, so that even when a temperature rise is caused by car fire, incineration of the gas generator and the like, the gas generant in the gas generator can be allowed to burn without any fracture of the container.
However, the container of the gas generator made of aluminum is significantly lower in strength in high temperature. When the gas generator made of aluminum is exposed to flame of the car fire and the like and the gas generants filled therein are burnt, if reduction in strength of the aluminum container itself is caused by the flame, then the container cannot withstand the burning pressure of the gas generants and thus can be burst. As a result of this, there is a possible fear that the fragments of the burst container may be flied off to the surrounding to injure the occupants and persons around them.
There have been proposed the countermeasures against the possible burst of the aluminum container, according to which an explosive composition that can ignite automatically at a temperature lower than the temperature at which reduction of strength of aluminum is caused is arranged in the aluminum container, in addition to the gas generant and the enhancer agent, so that the gas generant in the aluminum container is allowed to burn out before the reduction of strength of the aluminum container is caused, to avoid possible dangers of burst of the aluminum container and the like. It should be noted that the automatic ignitability of the automatically ignitable explosives means that the explosive can ignite within the range between 180xc2x0 C. and 210xc2x0 C. which is lower than the temperature at which the reduction of strength in high temperature of aluminum is caused.
For example, U.S. Pat. No. 4,561,657 proposed a system for an aluminum container wherein the explosive that can ignite automatically at a temperature lower than the temperature at which reduction of strength of aluminum is caused is arranged in close contact with an inner surface of the container. The automatically ignitable explosive used in this prior art uses nitrocellulose as a major component. Nitrocellulose itself lacks long-term stabilization under high temperature and, what is even worse, there is the possibility that it may spontaneously fire due to the deterioration. For this reason, it is necessary to use an additional automatically ignitable explosive composition, in addition to the gas generant and the enhancer agent. Thus, from the viewpoint of cost also, it is hard to say that the proposed system is of advantageous.
Further, Japanese Laid-open (Unexamined) Patent Publication No. Hei 4(1992)-265289, No. Hei 7(1995)-232989, No. Hei 8(1996)-508972 and No. Hei 8(1996)-511233 disclose automatically ignitable compositions, which however require that some structure therefor be provided in the interior of the gas generator or must be incorporated in the igniting charge or enhancer agent of the igniter, as is the case with the above-noted U.S. Patent. Thus, these prior arts have the disadvantages that the structure is complicated and that the cost is increased.
Republished Patent No. WO97/20786 discloses the enhancer agent having the automatic ignitability. However, this enhancer agent has the calorific value of 3,400 J/g which is lower than the calorific value of 6,700 J/g of BKNO3 and also includes little metal thermo-particles. Therefore, the enhancer agent of this prior art has the disadvantages that there is the possibility that an ignition lag of the gas generator and thus an output failure may be caused.
The conventional enhancer of BKNO3 has an ignition point of about 470xc2x0 C., around which the function of automatic ignitability defined herein is not provided, however.
In the light of the disadvantages mentioned above, the present invention has been made. It is the object of the present invention to provide the enhancer agent composition having automatic ignitability as well as a high calorific value.
The inventors have been devoted themselves to consider possible ways of solving the problems above and found that by defining the composition of the enhancer agent, the automatic ignitability of the enhancer was developed and also the high calorific value was produced, to thereby produce improved ignition of the gas generant. Deriving from the above, the inventors have accomplished the present invention.
From the examination of the probability of successful ignition of the gas generant resulting from the enhancer agent, it is found that it is preferable that the enhancer agent has the following characteristics i to iii. The present invention has been made on the basis of the understanding that the present invention is directed to an explosive composition that is fundamentally different in idea from the gas generant that seeks for a low calorific value and an increased amount of generated gas in this sense. Although it is ideal that the enhancer agent composition has all characteristic features i through iii listed below, it can function as the enhancer agent by the characteristic feature i only.
i. The enhancer agent composition has a high calorific value for supplying a sufficient calorific value to the gas generant for successful ignition of the gas gene rant;
ii. The enhancer agent composition has the required number of moles of the generated gas for generating an adequate gas flow to ignite the whole area of each individual gas generant; and
iii. The enhancer agent composition has lots of metal thermo-particles to adhere directly to the gas generant to ignite it
In other words, the auto-ignition enhancer composition of the present invention is characterized in that it comprises the following components and having a heating value of 4,500 J/g or more, or preferably 6,000 J/g or more:
(a) 5-aminotetrazole,
(b) a metal powder,
(c) at least one compound selected from the group consisting of potassium nitrate, sodium nitrate and strontium nitrate, and
(d) molybdenum trioxide.
The auto-ignition enhancer composition of the present invention provides the automatically ignitability by adding an oxidizing agent, such as potassium nitrate, sodium nitrate, and strontium nitrate, and molybdenum trioxide, in addition to 5-aminotetrazole and the metal powder. It should be noted here that the automatic ignitability means that the ignition takes place in the range of between 180xc2x0 C. and 210xc2x0 C. It can be said that this is a necessary temperature range from the viewpoint of the reduction of strength of aluminum in high temperature.
Since the auto-ignition enhancer composition of the present invention has a high calorific value of not less than 4,500 J/g, as well as the automatic ignitability, it has a suitable property as the enhancer agent composition. Particularly, the auto-ignition enhancer composition having a calorific value of 6,000 J/g or more is optimum as the enhancer agent composition.
As a result of this, the auto-ignition enhancer composition of the present invention can provide the automatic ignitability for the gas generator without complicating the structure of the gas generator.
Further, the auto-ignition enhancer composition of the present invention is characterized by combination of the feature above with the feature that the number of moles of the generated gas is in the range of 0.5 or more to 2.0 or less per 100 g of the ignitable enhancer agent composition. When the number of moles of the generated gas is in the range of 0.5 or more to 2.0 or less per 100 g of the enhancer agent composition, the gas flow suitable in amount for igniting the gas generants can be supplied thereto.
The auto-ignition enhancer composition of the present invention can obtain a well stabilized ignition performance by combining the high calorific value and the properly generated gas flow in a balanced manner. As a result of this, the auto-ignition enhancer composition of the present invention can develop the automatic ignitability, while keeping its superior inflammation to that of a general BKNO3 enhancer agent.
Though the metal powder that may be used include a powdered metal alloy, it is preferable that the metal powder is at least one material selected from the group consisting of aluminum, magnesium, magnalium, boron, titanium, and zirconium.
Specifically, when boron is selected for the metal powder, the auto-ignition enhancer composition of the present invention preferably has the following composition ratio I, or further preferably the composition ratio II:
I. (a) 5-aminotetrazole: 3 weight % or more to 25 weight % or less,
(b) boron: 5 weight % or more to 30 weight % or less,
(c) potassium nitrate: 50 weight % or more to 85 weight % or less, and
(d) molybdenum trioxide: 0.2 weight % or more to 10 weight % or less;
or
II. (a) 5-aminotetrazole: 5 weight % or more to 15 weight % or less,
(b) boron: 16 weight % or more to 25 weight % or less,
(c) potassium nitrate: 60 weight % or more to 80 weight % or less, and
(d) molybdenum trioxide: 1 weight % or more to 7 weight % or less.
This auto-ignition enhancer composition of the present invention contains a small amount of nitrogen-containing organic compound, i.e., 5-aminotetrazole, of 25 weigh %, or preferably 15 weight %, and a large amount of metal powder within the range between 5 weigh % and 30 weight %, or preferably between 16 weight % and 25 weight %. This can allow the gas generants to be ignited directly by a lot of metal thermo-particles, thus providing little temperature dependency and stable ignitable performance.